Aerial imaging typically employs an optical imaging and projection system to project a two-dimensional aerial image onto an image plane. The location of the image plane is defined by the optics of the imaging and projection system. The same optics also limits the imaging performance by introducing diffraction, aberration and other effects. A straightforward method of sensing the aerial image is to place a conventional image sensor (for example, a conventional CCD) at the aerial image plane. In this method, the resolution of the sensed image is limited by the sensor pixel size. Conventional CCDs generally have a resolution in the order of 2 to 20 micrometers.
In certain applications, the quality of the aerial image is very high. For example, in the field of photolithography, the aerial image is an image of a reticle or mask produced with a photolithographic device such as a “stepper” or a “scanner”. The stepper and scanner employ different stepping/scanning procedures while imaging the mask to produce a mask pattern in the image plane where a wafer to be patterned is located. Because the mask pattern contains some very small features (for example, with minimum feature size in the order of one tenth of a micron in VLSI (Very Large Scale Integrated circuit) technology), the aerial image sensing requires a very high resolution, for example, in the order of 50 nm.
Certain prior art describes various techniques for improving the monitoring and adjusting the imaging performance of the imaging systems. For example, A. Grenville, et al., “Image Monitor for Markle-Dyson Optics”, Journal of Vacuum Science and Technology B, No. 11, Vol. 6, November/December 1993, pp. 2700-2704 describe a scanned grating technique for quantifying the imaging properties of optical imaging and projection systems producing aerial images.
Nakagiri et al., U.S. Pat. No. 5,464,977 (“Nakagiri”) describes an optical detection apparatus and method for measuring, at a high resolution, an image in the wavelength range from the infrared to the gamma ray. Nakagiri uses a photoelectric conversion medium to produce a change in an electric property within a photoelectric conversion medium according to incident image light. A probe placed in contact with the medium scans the medium to measure the change in the electric property to derive distribution information corresponding to the image.
Examining masks for lithography in high-energy ranges of the electromagnetic spectrum, such as the X-ray range, presents additional challenges because of, for example, the severe limitations on imaging and lack of suitable transmission optics. In U.S. Pat. No. 6,002,740, Cerrina and Lucatorto propose a method and apparatus for inspecting X-ray and extreme ultraviolet (EUV) masks and other objects. This method employs a converter to convert the image produced by X-rays or EUV light incident thereon to an image formed by electrons emitted from the converter. The emitted electrons are magnified in an electron microscope by as much as 100 to 1,000 times and the magnified electron image is displayed by the electron microscope. The resolution obtained in the Cerrina and Lucatorto system corresponds to the resolution of the photoemission electron microscope and is in the range of 20 nm to 200 nm depending on the energy range of the emitted photo-electrons.
There is a need for system and technique that provides enhanced sensing resolution of an aerial image produced by conventional transmission optics of, for example, photolithography steppers or scanners. There is a need for an improved system and technique that enables sensing an aerial image produced by visible or ultraviolet optics at a resolution of, for example, 50 nm.
Moreover, there is a need for an apparatus and a method for aerial image sensing at enhanced resolution that may be employed for mask inspection, preferably in-situ, in a conventional photolithographic stepper or scanner. In this application, the sensing resolution is significantly smaller than the minimum feature size in the pattern to be printed on the wafer. Further, there is a need for an apparatus and a method that facilitates calibration, tuning and monitoring of the conventional photolithographic stepper or scanner using the sensed aerial image.